This invention relates to a transistorized power switch and base drive circuit therefor and in particular to a transistorized power switching circuit for controlling the conductive state of a Darlington transistor that provides a constant reverse base drive to switch the Darlington transistor to a nonconductive state wherein the number of components in the switching circuit that introduce time delays is minimized.
Advances in high power transistors has opened new applications in power electronics for transistors that were previously dominated by other solid state switches such as the silicon controlled rectifier. Power transistors have the advantages of higher frequency application, low switching losses and relatively simple commutation schemes as compared to the silicon controlled rectifier. High power transistors are now finding application in motor drives, inverters, uninterrupted power supplies and converters. In many applications the high power transistor employed is a power Darlington transistor. Power Darlington transistors are available that provide a collector to emitted operating voltage of 1000 volts at up to 300 amps to drive a load. The power Darlington transistor however does not achieve the switching speed performance of a lower current, narrow-base-width, transistor. The physical width of the base in a power Darlington must be increased for the power transistor to support higher voltages and currents. Increasing the base width lowers the current gain and therefore to provide the high gain desired a power Darlington configuration is employed.
The relatively poor switching performance of a conventional high powered Darlington transistor is most recognizable during the turn-off period. The relatively slow turn-off is due to the time required for charge entering the base of the first transistor in the Darlington pair passing through the first transistor thence through the second transistor of the Darlington pair to the load.
As an alternative to permitting the charge entering the base of the first transistor of the Darlington pair passing through the first transistor thence the second transistor of the Darlington pair to the load, an effective reverse bias may be used to drive the charge entering the base of the first transistor of the Darlington pair at the instant the Darlington transistor is being turned off back out through the base region of the first transistor of the Darlington pair. Most schemes developed to improve the slow turn-off of Darlington transistors employ five or six transistors each introducing a time delay or alternatively requiring two signal drivers to switch the Darlington transistor to a conductive or nonconductive state.
Various schemes have been developed to turn-off a power Darlington transistor more rapidly. Some of the schemes employ capacitors or inductors. Others require two power supplies such as a positive and negative power supply. What is needed is a circuit for switching a power Darlington transistor to the conducting or nonconducting states that minimizes the number of circuit components in which a time delay may be introduced while concomitantly providing a base drive that permits the full on time and delivers reverse biasing to the Darlington transistor during the full off time regardless of how long the off time is.